Artificial honeycomb structures are used in a wide range of applications, such as for structural support, impact absorption, filtration, acoustic dampening, chemical reaction catalysis, and heat storage and exchange. Through the present, honeycomb has been manufactured with either all substantially solid walls or all substantially porous walls, which has limited the performance and applicability of honeycomb type structures in a number of ways.
One important field in which the use of honeycomb has been restricted due to its limitations is heat exchange, with other articles more commonly employed today in thermal applications.
The use of structures and materials to promote and control the exchange of heat from one body to another is of great importance in a number of industries. Heat exchanging devices are of vital importance in numerous fields, ranging from the field of energy production, HVAC (Heat, Ventilation, and air conditioning), computing and other electronic devices, mechanical devices, and chemical processing, to the field of food preparation and storage.
For example, the increasing power demands and decreasing size of computing and electronic components and devices place a premium on compact and efficient thermal control systems. Regulation of temperature beneath critical thresholds can enhance the function and efficiency of, and prevent damage to, key electronic components.
Numerous means of controlling temperature have been utilized, including the use of honeycomb, fin arrays, pin fin arrays, metallic foams, and other structural elements, and the incorporation of different materials with various heat transfer properties. In electronic systems, air is generally used as the fluid medium into which heat exchangers dissipate excess heat. Because of the low thermal capacity of air, the movement of heat into air is commonly a rate-limiting step. It is well known that a high heat transfer area between exchanger and air is one of the most important means for mitigating this problem and achieving rapid heat dissipation. Metallic foams achieve high heat transfer areas in compact structures due to their high surface area to volume ratios, and have recently been proposed or adapted for use in numerous heat exchange applications, including in the field of electronics, as disclosed, for example, in U.S. Pat. Nos. 6,840,307 and 6,761,211. However, due to tortuosity effects, the effective thermal conductivity of foamed metal can commonly be under ten percent of that of the base material. Metallic foams have been combined with fins into composite heat exchangers to combat this problem, but the multiple components required for such structures may lead to difficulty and expense in manufacture, especially if the alternating components are densely packed in an attempt to optimize the balance of conduction and heat transfer area. Furthermore, metallic foams themselves may be expensive to manufacture.
More traditional structures, such as fin arrays and pin fin arrays, do not suffer from reduced effective thermal conductivity, due to the solid construction of fins and pins and the direct paths for heat conduction thus afforded. However, fin arrays and pin fin arrays typically achieve significantly lower surface area to volume ratios due to their relative simplicity of structure and the relatively thick fins generally required for inexpensive manufacture and structural integrity. Example fin embodiments are illustrated in U.S. Pat. Nos. 6,273,186 and 6,397,931, and U.S. Patent Application No. 2006/0092613 A1. While these devices and materials have been useful in providing a means of controlling the temperature of a given body or fluid, they have been of limited use in providing advantageous and inexpensive combinations of effective thermal conductivity away from a heat source, and overall surface area to volume ratio.
In addition, traditional honeycomb structures and vented honeycomb structures have been used for some heat exchange applications. However, as they do not allow significant transverse fluid flow through their cell walls they are not well-suited for continuous or high-performance heat exchange tasks. Porous honeycomb structures, such as the extruded and sintered integral honeycomb described in U.S. Pat. No. 6,881,703, may be more suitable, as they allow transverse flow. However, general methods of manufacturing porous honeycomb (such as sintering) can be very expensive, and have practically limited the manufacture of porous honeycomb to small blocks of integral honeycomb. The traditional methods of manufacture can also require a minimum wall thickness that is generally relatively thick, leading to relatively poor surface area to volume ratios. These methods can also produce very small and sinuous pores and result in high pressure drops in transverse flow. Furthermore, embodiments in which the entire honeycomb is porous-walled lack key advantages of solid walls, including structural strength and increased speed of heat conduction along cellular column axes.
Traditional honeycomb structures may also generally be very strong, and can therefore absorb a significant amount of mechanical energy. However, this traditional honeycomb often yields catastrophically once a sufficient pressure is applied. Honeycomb must, in certain cases, be pre-crushed for energy absorption applications where smooth absorption is important, which may be wasteful and imprecise. Traditional honeycomb can also provide an exceptionally high strength-to-weight ratio for structural applications. However, for applications where available honeycombs provide an excess of strength, weaker materials with lower weight may be desirable.
Porous honeycomb has been used in filtration applications (e.g. in U.S. Pat. No. 4,329,162), but pore size is generally uniform in existing porous honeycomb embodiments. As a result, porous honeycomb has not been used to enable progressive filtration of differently sized particles.